The present invention relates to a tire comprising a carcass ply, an inner elastomeric layer for sealing against the inflation gases and an elastomeric reinforcement layer between said ply and said inner layer. The invention is particularly applicable to tires of radial structure for motor vehicles bearing heavy loads, in particular for heavy vehicles and construction vehicles.
It is known to the person skilled in the art that a radial carcass tire for a motor vehicle bearing heavy loads has a framework comprising the following elements:                (i) one or more bead wires in the bottom zone of the sidewalls,        (ii) a carcass reinforcement extending from one bead wire to the other and comprising a carcass ply radially and axially to the outside of the inner layer, said ply being formed of metal cords coated in an elastomeric layer, and        (iii) a crown reinforcement, radially located to the outside of the carcass reinforcement, which comprises at least two crown plies.        
The skeleton is consolidated and protected by elastomeric layers arranged on either side of the carcass ply, as follows.
Axially to the outside of the carcass ply, the sidewalls function to reinforce the carcass reinforcement and protect against mechanical and/or chemical attack to which the carcass ply is subjected.
Axially and radially to the inside of the carcass ply is an inner elastomeric layer which defines the radially inner face of the tire. The inner layer surrounds and circumscribes the inner space of the tire into which inflation gases are administered. This inner layer is made sufficiently airtight with respect to the inflation gases to maintain the inflation pressure within the tire during travel. The inner layer protects the carcass ply from the migration of the inflation gases, in particular oxygen, which are under high pressure. The inner elastomeric layer is generally formed from a composition based on halogenated butyl rubber.
An elastomeric reinforcement layer between said carcass ply and said inner layer is provided to reinforce the inner layer, as well as the carcass ply. This intermediate reinforcement layer provides mechanical connection of the carcass ply to the inner layer, and is also intended to delocalize the forces to which the carcass reinforcement is subjected during use of the tire (i.e., travel).
The reinforcement layer also protects the carcass ply from attack, such as the diffusion of oxygen, so that the ply retains its original characteristics for as long as possible. Thus, the intermediate reinforcement layer increases the life of the tire and the number of possible recappings thereof.
It is essential that the intermediate reinforcement layer have satisfactory compatibility and, especially, mechanical connection, with both the inner layer and the carcass ply in order to avoid separation during travel.
Preferably, the composition for the intermediate reinforcement layer is close to that used for the carcass ply, so that the constituents that promote adhesion between the elastomer and metal cords of the ply do not migrate towards the neighbouring layers and that constituents of the neighbouring layers which might adversely affect adhesion do not migrate in the opposite direction towards the carcass ply. When the compositions of the intermediate layer and the carcass ply are very close, e.g., have the same elastomeric base, in the absence of a concentration gradient and a difference in solubility, there is no possibility of migration of the constituents.
It is also known that reducing the temperature of the inner layer during travel provides for increased life, by slowing down thermal and thermo-oxidizing aging.
There are various ways of effecting this reduction in temperature during travel, including using compositions of low hysteresis or reducing the thickness of the tire to promote dissipation of the heat towards the outer surface, with the advantage of reducing the suspended mass of the vehicle.
However, in order for the intermediate reinforcement layer to be an effective reinforcement for the carcass ply and to provide a barrier to oxygen with respect to the carcass ply, the layer must have a sufficiently great thickness, which also promotes the consumption of oxygen which has migrated across the inner layer.
On the other hand, if an increase in the temperature of the internal layer during travel benefits this oxygen-barrier effect, it also benefits the migration of oxygen. Hence, the final result is virtually unpredictable to the person skilled in the art.
In summary, the intermediate reinforcement layer must provide the following:                withstand, without damage, the high elongations that take place in the uncured state during the shaping phase of the manufacture of the tire, which implies high cohesion in the uncured state;        contribute to the reinforcement of the carcass ply, both in the uncured and in the cured state, by delocalizing part of the mechanical forces to which the ply is subjected during travel;        provide mechanical connection between the carcass ply, generally based on natural rubber and the inner layer, generally based on a halogenated butyl rubber;        not adversely affect the composition of the carcass ply before, during or after vulcanization, by a composition insufficiently close to that of said ply; and        be a barrier to the migration of oxygen which has passed through the inner layer.        
There are various ways of limiting the migration of oxygen through the intermediate reinforcement layer including:                a) increasing the thickness of the layer, which has the disadvantages of increasing the weight and cost of the tire and increasing the heating of the internal layer during travel,        b) reducing the partial oxygen pressure in the inner space of the tire by inflating it with an air mix that is low in oxygen, as described in patent specification JP-A-10 258 604 or, alternatively, by providing the wheel with a means for absorbing oxygen, as described in patent specification JP-A-08 230 421,        c) fixing the oxygen in the intermediate reinforcement layer by reducing the amount of antioxidant which, however, does not contribute to a long life of the carcass reinforcement,        d) using, as indicated in ENCYCLOPEDIA KIRK-OTHMER, third edition, Wiley, volume 3, page 483 in the chapter “Barrier Polymers”, a polymer less permeable to oxygen that has been made so by increasing its glass transition temperature Tg, its polarity, its crystallinity, its chain rigidity, its degree of compactness (order, symmetry), or alternatively by using in the highest amount possible a coarse and/or lamellar filter which interacts satisfactorily with the elastomer (e.g. via a filler/elastomer coupling agent). However, this amount of filler is limited by the requirements of working and of fatigue resistance under repeated stresses.        
In order to achieve an improvement in the life of tires bearing heavy loads, the person skilled in the art can alter numerous physical, chemical and physico-chemical parameters, while taking into account the requirements of rolling resistance, weight and cost of the tires. These improvements can be achieved by carrying out tests of tires travelling on a vehicle, but such tests are costly. Moreover, the results of the tests are not readily predictable because of the number of parameters to be considered with the interactions thereof being numerous and not well defined.